Conventional optical fibers made of quartz, which is capable of providing good optical transmission across a broad wavelength spectrum, have been in practical use mainly for trunk lines. Such quartz optical fibers, however, are expensive and poor in workability. Accordingly, plastic optical fibers (hereinafter, abbreviated as “POF”), which offer advantages such as being much more inexpensive and lightweight, having larger apertures and easily workable end surfaces, and being easily handled, have been in practical use for lighting, sensors, and interior wiring such as FA, OA and LAN.
Among them, a step-index (SI) POF having a core/cladding structure using a poly(methyl methacrylate) (PMMA) as the core material and a low refractive-index fluorine-containing olefin copolymer as the cladding material has gradually been in practical use in the form of a POF cable having coating layer(s) on the outer surface of the POF, for in-vehicle LAN communication wiring because it is available for high-speed data communication and would be a better choice in terms of weight reduction, cost reduction in communication systems, and anti-electromagnetic noise measures.
When used in an automobile where the ambient temperature reaches around 100 to 110° C., a POF cable of the aforementioned type is required to have good heat resistance. In particular, when installed in a high-temperature environment such as in the vicinity of the engine where there are oil, electrolyte, and flammable substances such as gasoline, the POF cable is required to excel not only in heat resistance but also in chemical resistance. From this standpoint, there have been proposed many techniques in which polyamide resins (nylon-based resins) such as nylon 11, nylon 12, nylon 6/12, nylon 6, nylon 66, and nylon 6/66, which have good heat resistance and chemical resistance, are used as the coating material of the POF cable.
For example, Patent Document 1 (Japanese Patent Laid-Open No. 10-319281) and Patent Document 2 (Japanese Patent Laid-Open No. 11-242142) each proposes a POF cable including a POF, and a primary coating layer made of a black polyamide resin and a secondary coating layer made of a colored polyamide resin that are formed around the POF. Given examples of the polyamide resins include nylon 6, nylon 11, and nylon 12.
Patent Document 3 (International Patent Publication No. WO 01/48526) and Patent Document 4 (Japanese Patent Laid-Open No. 2003-315638) each discloses a POF cable including a bare POF, and an adhesive layer made of a material containing a polyamide-based polymer, a primary coating layer and a secondary coating layer that are sequentially formed around the POF. Given examples of the polyamide-based polymers include nylon 6, nylon 66, nylon 11, and nylon 12.
Patent Document 5 (Japanese Patent Laid-Open No. 2003-255202) proposes a POF cable including a bare POF and a coating layer formed around the POF, the coating layer being made of a polyamide resin containing an inorganic pigment based on a rare metal. Given examples of the polyamide-based resin include polyamide 11, polyamide 12, polyamide 6/12, polyamide 66, and polyamide 66/6. Patent Document 6 (Japanese Patent Laid-Open No. 2004-226925) proposes a POF cable including a bare POF and a coating layer formed around the POF. The coating layer is made of a polyamide resin containing an inorganic ultramarine blue pigment. Given examples of the polyamide resin include nylon 11, nylon 12, nylon 6, and nylon 66.
Patent Document 7 (Japanese Patent Laid-Open No. 2000-231045) describes a POF cable including a bare POF, and a primary coating layer and a secondary coating layer that are formed around the POF. The primary coating layer is made of a nylon-based resin having a melting point of 200° C. or lower, and the secondary coating layer is made of, for example, a vinyl chloride resin or nylon 12 having an oxygen index of 25 or higher.
However, POF cables having a coating layer made of a polyamide resin (nylon-based resin) material can suffer from problems as follows.
Typical polyamide resins such as nylon 12 are industrially synthesized by polycondensation of amine and carboxylic acid. However, the polymerization of a polyamide resin is accompanied by the establishment of a chemical equilibrium, so that the resultant polymer is always contaminated with a monomer or oligomer derived from a feed material for the polyamide resin.
Investigations made by the present inventors have demonstrated that transmission losses in POF are significantly increased for POF cables wherein a primary coating layer made of a polyamide 11, polyamide 12, or polyamide 6-12 resin is provided in contact with a bare POF, and for POF cables having a secondary coating layer made of these polyamide resins, as described in the aforementioned Patent Documents, when the cables are left in a high temperature environment at 100° C. or higher for a long period of time.
The present inventors have analyzed a possible cause of this in depth. As a result, the inventors have ascertained that the residual monomers or oligomers derived from the feed material are responsible for the aforementioned increase in transmission loss in POF, that is, they dissolve and diffuse into the bare POF from the primary and/or secondary coating layer(s) to cause the increase in transmission loss.
In addition, it has been found that particularly significant increase in transmission loss is observed when the outermost cladding layer is made of a fluorine-containing olefin resin having a tetrafluoroethylene (TFE) unit and when heat of crystal fusion is greater than a certain amount of value.
Examples of the aforementioned monomers derived from a polyamide resin material include aliphatic diamino acid compounds, aliphatic dicarboxylic acid compounds, and amino-aliphatic carboxylic acid compounds that form polyamide resins. More specifically, examples include 11-aminoundecanoic acid for nylon 11, 12-aminododecanoic acid for nylon 12, hexamethylene diamine and dodecanedioicate for nylon 6-12, hexamethylene diamine and sebacate for nylon 610, ε-aminocaproic acid for nylon 6, hexamethylene diamine and adipic acid for nylon 66, 1,10-decanediamine and 1,12-dodecanediamine for nylon 1010, and 1,12-decanediamine and 1,12-dodecanedioic acid for nylon 1012. Examples also include cyclic lactam compounds having an endocyclic amide bond (—CONH—) obtained through intramolecular cyclic esterification of the molecular chain terminals of an aminocarboxylic acid compound. Specific examples include ε-caprolactam for nylon 6 and lauryl lactam for nylon 12. It is noted that the monomers derived from the feed material as used herein include low-molecular-weight compounds produced as by-products during the synthesis of the feed material.
On the other hand, examples of the aforementioned oligomers derived from a polyamide resin material include compounds having molecular chain terminals that has an amino group (—NH2) and/or a carboxyl group (—COOH), which are formed through intermolecular esterification of the molecular chain terminals of two or more molecules of the aforementioned feed monomers (e.g., aliphatic diamino acid compounds, aliphatic dicarboxylic acid compounds, and amino-aliphatic carboxylic acid compounds, as described above) in the course of the condensation polymerization for the production of the polyamide resin; cyclic lactam compounds having an endocyclic amide bond (—CONH—) formed through further intramolecular esterification of the molecular chain terminals of the above compounds; compounds formed through intermolecular esterification of the above compounds; and compounds formed through an intramolecular/intermolecular secondary reaction (deamination reaction or decarboxylation reaction).
When the aforementioned monomers and oligomers are linear, the terminal amino group has high affinity with fluorine-containing olefin polymers, and the monomers and oligomers thus tend to stay in the cladding layer made of the fluorine-containing olefin polymer. This often causes reduction in transparency of the cladding material, which can result in significant deterioration of transmission characteristics of the POF cable. Meanwhile, when the aforementioned monomers and oligomers are cyclic lactam compounds, the monomers and oligomers tend to migrate to the vicinity of the interface on the inner layer side of the cladding layer (the core or the first cladding layer side) to form particulate structures. As a result, more structural mismatch would happen at the core-cladding interface or a cladding-cladding interface if there are two or more cladding layers with a tendency of significant deterioration of transmission characteristics of the POF cable.
Among the aforementioned oligomers, those having a lower molecular weight tend to dissolve and diffuse into POF more easily. In particular, those having a molecular weight of 2,000 or lower have a remarkable tendency of it.
As described above, the POF cables are required to have a good heat resistance. In particular, the POF cables that are intended to be used in an automobile are required not to cause any significant increase in transmission loss for a long period of time of longer than 5,000 hours, in an environment at 105° C. However, it is difficult for the conventional POF cables as described in the aforementioned Patent Documents to meet the required performance because increase in transmission losses is inevitable for the aforementioned reasons, after having been placed in a high temperature environment for a long period of time.
In addition, it has been reported that optical properties of a POF may be significantly degraded due to excessive stress on the POF when the POF is coated with a nylon-based resin such as nylon 66 having a relatively high melting point. For example, Patent Document 7 (Japanese Patent Laid-Open No. 2000-231045) discloses a POF cable having a POF around which a primary coating material of a nylon 66 resin is directly provided, as comparative examples (Comparative Examples 2 and 8) to the proposed invention. The document discloses that, in the POF cable, the polyamide 66 resin having a high melting point is directly coated over the bare POF at a high coating temperature, which brings about deformation of the POF and increase in transmission loss, and hence nylon-based resins having a high melting point are not suitable as a coating material for POFs.
Considering now light emitting diodes (LEDs) which have been used as a light source for POFs, those with a center emission wavelength of around 650 nm are most commonly used but currently cannot provide sufficient long-term heat resistance at 100° C. or higher. The reason is that such LEDs are made of a GaAlAs-based material and their aluminum component, if contained too much, tends to lower the LED's own heat resistance.
As a signal transmission system having an excellent heat resistance at 100° C. or higher, Patent Document 8 (Japanese Patent Laid-Open No. 2001-74945) discloses a signal transmission system including an LED with a center emission wavelength of 930 to 990 nm and a POF whose core is made of a norbornene-based resin, and Patent Document 9 (Japanese Patent Laid-Open No. 2001-21737) discloses a signal transmission system including an LED with a center emission wavelength of 750 to 850 nm and a POF whose core is made of a polycarbonate resin. LEDs of which center emission wavelength falls within a near infrared region contain less aluminum component, and is thus superior in heat resistance at 100° C. or higher. In addition, POFs which are placed in such a high temperature environment typically undergo increase in electronic transition absorption due to thermal oxidative deterioration of a bare POF and increase in Rayleigh scattering due to migration of a low-molecular-weight compound contained in a coating material into the bare POF. The value of transmission loss within the near infrared region is hardly affected by them. Accordingly, in the signal transmission system described in the aforementioned Patent Documents, the transmission loss is less changed with time even in a high temperature environment at 100° C. or higher, and therefore, it is possible to keep the transmission loss at a constant level over a significantly long period of time.
However, transmission losses are as high as the 6,000 s dB/km at 930 to 990 nm in the POF whose core is made of a norbornene-based resin as described in the aforementioned Patent Document 8, while transmission losses are 1,000 s dB/km at 750 to 850 nm in the POF whose core is made of a polycarbonate-based resin as described in Patent Document 9. As apparent from the above, the transmission losses in the POFs described in these Patent Documents are too high to use them in practice for in-vehicle LAN communication wiring.
On the other hand, known as visible light emitting diodes having a center emission wavelength at or below 600 nm are InGaN-based LEDs (center emission wavelengths of 505 nm and 520 nm), PGaN-based LEDs (center emission wavelength of 565 nm), and InGaAlP-based LEDs (center emission wavelength of 590 nm). They contain no or a small percentage of, if any, aluminum component that otherwise would cause reduction in heat resistance of the LED. This means that the heat resistance of these LEDs at 100° C. or higher comes to the point where these LEDs can be put into practical use. Furthermore, in the POFs whose core is made of a PMMA resin, there is a wavelength window around 570 nm where the transmission loss is 80 to 90 dB/km, which is remarkably lower than the transmission loss in a wavelength window around 650 nm where the transmission loss is about 130 to 140 dB/km.
However, the transmission loss in a POF is likely to increase in such short-wavelength regions when the POF is placed in a high temperature environment at 100° C. or higher because it tends to be affected by the aforementioned increase in electronic transition absorption or Rayleigh scattering. It has therefore been believed to be difficult to practically use a signal transmission system including a POF and an LED having a center emission wavelength in a range of 500 nm to 600 nm, in a high temperature environment such as in an automobile.